Why use cover crops? • Cover crops are an important fertility management tool available to organic farmers • Grown primarily for soil or ecosystem improvement rather than cash • Can have negative consequences if managed incorrectly or the wrong species are chosen.
Terminology • Confusion about the terms: • cover crop, green manure, and catch crop. • Frequently used interchangeably, but refer to different primary functions: • Cover crop = used to prevent soil erosion by covering soil with living plants • Green manure = turned under for soil improvement • Catch crop = used to “catch” nutrients left after harvest of a cash crop and prevent leaching
Goal: Increase nutrient availability • Cover crops can increase nutrient availability by: • residue breakdown - releases nutrients into soil solution or incorporated in soil microbes. • As a source of readily available C cover crops stimulate microbial activity and increase the breakdown of soil OM. • If cover crop is deeper rooted than preceding crops it can recycle nutrients from deeper in the soil and return them to the upper soil when residue is incorporated • Cover crops rather than bare fallow reduce nutrient loss by capturing nutrients vulnerable to leaching and preventing soil erosion.
Goal: Increase SOM and improve nutrient availability • To build SOM look for a high biomass cover crop. • Possible options for fall planting include non-legumes such as: • annual rye grass, cereal rye, triticale, legume/cereal mix, • High biomass legumes provide N and build SOM: • sub-clover or woollypod vetch • For summer planting options include: • sorghum/sudan grass, buckwheat • tropical legumes: • cowpea, crotolaria, pigeon pea, lablab bean,sesbania
Lablab beans cowpeas Summer cover crops - tropical species Sorghum (C4 plant) Pigeon peas
Timing of nutrient release from the cover crops depends on a variety of factors • Soil temperature and moisture affect microbial activity (lower when cool, dry or waterlogged) • “quality” of the cover crop residue also important: • C to N ratioof the residue determines whether there is net release of N (mineralization) or a net decrease in available N due to immobilization by the soil microbes. • C:N ratios around 22 or less lead to net mineralization, • C:N ratios above 22 lead to net immobilization of N. • At low C:N ratios microbes have more N than C available for growth, so they release N during decomposition. • At high C:N ratios soil microbes have more C than N available and take up N from the soil solution • lignin, tannins and polyphenols in residue are less palatable to microbes and slow residue decomposition.
Effect of C:N ratio on size and timing of peak soil nitrate levels over 3 years - UC Davis field (yr 1, yr2, yr 3)
Goal: Provide N • Here the best choice would be a legume that is well adapted to your area. • It is important to ensure that sufficient rhizobia bacteria are present in the soil to give good root nodule formation, and hence N2 fixation. • May need to inoculate with rhizobium if it is the first time a particular legume cover crop has been grown in a field.
Timing of nutrient release • Important to manage the timing of nutrient release from cover crops with times of high crop demand • otherwise nutrients become vulnerable to loss through leaching and the crop may not receive adequate nutrition at key stages in the growth cycle.
How to roughly estimate N contribution from a cover crop • Calculate above ground dry weight for a given area: • a) Take a number of samples from the field (clip at ground level, using a yardstick or frame to measure area to be sampled) • b) Dry for a few days in sun, greenhouse or oven (140F) until "crunchy" or brittle. • c) Calculate the dry weight produced in lbs/ac as follows: • Dry wt (lb/ac)= wt of samples x 43,560 sq.ft • # of sq. ft sampled • Multiply dry wt by the % N content of the biomass to give total N in cover crop in lbs/ac: • Total N (lb/ac) = dry wt x %N 100
Typical % N contents for cover crop types • Before flowering: • Annual legumes 3.5-4% • Perennial legumes 2.5-3% • Grasses, brassicas 2-3% • When flowering: • Annual legumes 3-3.5% • Perennial legumes 2-2.5% • Grasses, brassicas 1.5-2.5%
To estimate how much of the N will be available to the crop that season • If conventionally tilled: • divide total N by 2 • If left on surface: • divide by 2 in warmer climates • divide by 4 in cooler climates.
Goal: improve soil physical properties • Generally improve soil physical properties by: • increased SOM • increased microbial activity and production of extracellular “glues” that enhance aggregate stability • increased water infiltration due to increased porosity - both macro and micropores • It may be beneficial to include a non-legume to provide organic material that breaks down more slowly than a legume alone • Some deep-rooted species can help to break through compacted layers in the soil and improve drainage. • E.g. Sorghum-sudan grass • In a recent study cover crop use in moderately saline soils impaired soil physical properties
Goal: Weed suppression • Achieved by either: • outcompeting weeds by rapid canopy development and more vigorous growth • production of allelopathic compounds • provision of dense mulch • Examples: • good options include triticales, sorghum/sudan and other cereals, brassicas such as rapeseed and oilseed radish and high biomass or allelopathic legumes. Also a well-balanced mix can also work providing the canopy closes quickly • cereal rye is also effective due to combination of a dense canopy and allelopathy, but can become a problematic weed itself in small grain systems. • For effective mulches use species/mixes that produce lots of slowly decomposing biomass (have a relatively high C:N ratio)
What to avoid…... • Some legumes such as vetch produce “hard seed” that remains viable in the soil for years, and can become a weed if allowed to go to seed prior to incorporation. • Just as crop rotation is important it may be desirable to avoid using the same cover crop every year, particularly a single species. • Otherwise populations of weeds most competitive with that species will build up, as well as pests and disease organisms that also attack that species.
Goal : Provide mulch to conserve soil moisture • As for weed suppressive mulches look for a combination of high above ground biomass and moderate or high C:N ratio residues. • Most legume residues with their high nitrogen content will decompose too rapidly to be effective.
Goal: Scavenge nutrients & prevent leaching • To maximize nutrient scavenging the cover crop should have an extensive root system that develops quickly after planting. • Non-legumes such as small grains, cereal rye, triticale, rapeseed, annual rye grass oilseed radish and mustards work well, but some legumes are also suitable.
Goal: Prevent soil erosion • Here the key is to choose a species that rapidly covers the soil surface. • Many of the species that are good nutrient scavengers also provide excellent ground cover. • But... while annual rye grass is a good nutrient scavenger, it has fine leaves and is slow to cover the soil surface, and not a good selection for reducing erosion.
Goal: Protect water quality. • This is achieved by selecting species that both prevent soil erosion and scavenge nutrients during periods of high rainfall. • It is also important to avoid turning in high N cover crops when the soils will be vulnerable to leaching
Goal: reduce disease and pest severity • Effects of cover crops depend on the species used and timing of incorporation and planting of the subsequent crop. • Some species such as cereal rye, triticale, forage rapeseeds, mustards and oil seed radish are known to suppress certain plant parasitic nematodes and soil borne diseases, whereas most legumes are highly susceptible and can increase nematode populations. • Need to have information on pests and diseases prevalent in an area to identify which cover crops should be avoided, or only used in mixtures.
Goal: Provide habitat for beneficial insects and spiders • From work in orchards and vineyards it is clear that cover crops provide habitat for beneficials, due to: • vegetative cover • by providing food sources such as extrafloral nectaries (e.g. vetch) or flowers • This aspect of cover crop ecology has not been well studied for annual systems, and there may be more potential than has been recognized. • The key in annual systems is to make sure that when the cover crop is turned in there are alternative habitats for the beneficials to move to: • Field margin vegetation, strips of undisturbed cover crops or insectary plantings
Water issues • If wet spring can help dry out soil profile • Or • Delay early plantings because of need to incorporate (early plantings often most lucrative) • If have limited water need to carefully consider trade-offs of using cover crops • Timing critical • Use species with high water use efficiency
How to choose a cover crop • Once you have decided it is desirable to include a cover crop in a given production system there are 3 basic steps to follow: • Identify what you want the cover crop to do. • Identify the planting windows where the cover crop can fit in your crop rotation and what the climatic and soil conditions are at that time. • Once these two questions are answered, then you can select the best species or mix to be used.
Step 1. Identify what you want the cover crop to do? • Address the most important factors limiting the productivity and sustainability of your production system. • Possible goals include: • Improve nutrient availability and provide nitrogen • Improve soil physical properties • Reduce erosion, leaching and protect water quality • Suppress weeds, pests and diseases • provide beneficial habitat • provide surface mulch
Step 2: Identify where the cover crop fits in your crop rotation • Examine cash crop rotations to identify “windows” where cover crops can fit • Most cover crops are planted in the fall to provide cover over the winter months • In summer when temperatures are high, fast growing species such as sorghum/sudan, cowpeas, and buckwheat and other can provide a good biomass return in a short growth period. • It is critical to minimize conflicts in timing of field operations for cover crops and cash crops. • Once the window is identified then the species selection will depend on the climatic and soil conditions during the window. e.g.- frost patterns, soil and air temperatures for germination and growth, soil pH..
Step 3: Select species/mix to meet the goals and requirements from steps 1 & 2. • the final step is to match the potential candidates identified in step 1 with the required characteristics identified in step 2. • Remember - it is as important to consider the characteristics you don’t want as well as those you are looking for. • It is rare that the “perfect” cover crop will exist and trade-offs will need to be made between different goals. • A final consideration will be the cost and availability of seed, and the number and types of field operations required for the different options to make a sound economic assessment of the alternatives.